fe: VARIATION, DISTRIBUTION, AND EVOLUTION OF THE GENUS PARTULA. 
bicolor. The distinctive feature of the class is the tinted spire, which ranges in color 
from light orange through orange-red to orange-red-brown (figs. 25 to 35, plate 12) 
or “dull scarlet,” to use Pilsbry’s phrase. The inner surface of the shell is 
lighter than the outer. A yellowish or yellowish-brown tinge extends over the 
flaring lip in most instances, as in bicolor. When the shell is decorticated (figs. 
32, 33, plate 12) the spire is dull scarlet rather than orange-red. The apical colors 
are always lighter in adolescent individuals and deepen with advancing age (figs. 
34, 35, plate 12). 
In the Lolo colony, the mitella shells are uniformly more pallid, owing to the 
reduction of the yellow element (figs. 36, 37, plate 12). More rarely in other 
localities the ground-color is deeper yellow-brown, and in such cases the color of the 
spire is correspondingly intensified (figs. 38, plate 12); the specimen of the illustra- 
tion is from Dededo, and by way of contrast a fully decorticated and white-lipped 
mitella from the same area is figured (fig. 39, plate 12). 
Wherever mitella occurs in the southward territories, its general ground-color 
is less yellowish and more deeply brown, while the spire also is darker, verging 
toward purplish chestnut (fig. 40, plate 12); if such a shell should become decor- 
ticated, its body-whorl is a peculiarly dull and attenuated red in color, thus sug- 
gesting the distinct type dealt with later as mitella-rubra (fig. 41, plate 12). Finally, 
at Salifan are found mitella shells which are invariably dull corneous brown with 
deeply colored, purple-brown spires (fig. 42 to 44, plate 12). Thus the southernmost 
shells of this class are the most extreme variants in the direction of deepened color, 
as compared with the northernmost Tarague shells, which are the lightest and 
yellowest; but the colonies that lie between do not display any consistent inter- 
gradations from the one to the other extreme. 
The salient feature of the geographical distribution of mitella is that this 
class is the most abundant and the most widespread. Taking the island as a whole, 
more than 54 per cent of all adult shells fall within its bounds; this is all the more 
significant when we recall that there are no less than six other primary color-classes 
in the species. While mitella predominates in the majority of the local collections, 
there are exceptions, as at Santa Rosa and Lolo; its absence from the Asados area 
is anomalous. Toward the south, at about the Salifan-Ylig transverse zone, 
mitella is the sole representative of its species; still further south it disappears, 
so far as the actual collections are concerned, and it is replaced by sparse numbers 
with the bicolor pattern. 
mitella-rubra—The present group is less clearly defined than any other; only 
after repeated efforts to devise a more satisfactory arrangement of its members 
has it finally been established as a color-order, closely related to mitella, but dis- 
tinguished as mitella-rubra on account of the reddish suffusion over the last whorl 
(figs. 45 to 56, plate 12). While it may not be a unitary group like the others, yet 
for descriptive purposes we may accord it ordinal status. 
At one extreme we have the shells of Macajna second (figs. 45 to 48, plate 12), 
which are fundamentally like mitella, but the older part of the last whorl is clouded 
with red color, diffused or localized, While in a sense this terminal suffusion is an 
